In a world of vanishing resources such as coal, oil, and natural gas, it is not only important to be more efficient in our use of energy, but also to improve on our abilities to efficiently extract energy from renewable resources such as hydroelectric, wind, geothermal and solar. To eventually replace fossil fuels with renewable sources of energy it must be feasible to do so. Currently renewable power conversion systems must operate at higher efficiencies if they are to provide the huge amounts of energy consumed by humans on the planet.
There are currently several types of heat engines, each with its own characteristics and cycles. These heat engines include the Otto cycle engine, the Diesel cycle engine, the Brayton cycle engine, the Rankine cycle engine, the Stirling cycle engine, the Erickson cycle engine, and the Carnot cycle engine. A brief description of each engine is provided below.
The Otto cycle engine is an inexpensive, internal combustion, low-compression engine with a fairly low efficiency. This engine is widely used to power automobiles.
The Diesel cycle engine is a moderately expensive, internal combustion, high-compression engine with a high efficiency that is widely used to power trucks and trains.
The Brayton cycle engine is an internal combustion engine that is generally implemented with turbines and is generally used to power aircraft and some electric power plants. The Brayton cycle features very high power density, normally does not use a heat exchanger, and has a lower efficiency than the other cycles.
The Rankine cycle engine is an external combustion engine that is generally used in electric power plants. Water is the most common working fluid.
The Erickson cycle engine uses isothermal compression and expansion with constant-pressure heat transfer. It may be implemented as either an external or internal combustion cycle. In practice, a perfect Erickson cycle is difficult to achieve because isothermal expansion and compression are not readily attained in large industrial equipment.
The Carnot cycle engine uses isothermal compression and expansion and adiabatic compression and expansion. The Carnot cycle may be implemented as either an external or internal combustion cycle. It features low power density, mechanical complexity, and difficult-to-achieve constant-temperature compressor and expander.
The Stirling cycle engine uses isothermal compression and expansion with constant-volume heat transfer. It is almost always implemented as an external combustion cycle. It has a higher power density than the Carnot cycle, but it is difficult to perform the heat exchange, and it is difficult to achieve constant-temperature compression and expansion. A Stirling engine is a commonly used piece of equipment for power conversion and processing and has plenty of room for improvement. Currently one of the most promising configurations of a solar power conversion system is that of a Stirling engine attached to the focal point of a mirrored parabolic dish pointed at the sun. The mirrors focus the sun's radiation on the hot side of the Stirling engine to produce mechanical power which is fed to an alternator to create electricity at high efficiencies. Even so, the Stirling engines used are not necessarily operating at their full efficiency potential.
The Otto cycle, the Diesel cycle, the Brayton cycle, and the Rankine cycle all have efficiencies less than the maximum because they do not use isothermal compression and expansion steps. Further, the Otto and Diesel cycle engines lose efficiency because they do not completely expand high-pressure gases, and simply throttle the waste gases to the atmosphere.
The Stirling, Erickson, and Carnot cycles are as efficient as nature allows because heat is delivered at a uniformly high temperature, Thot, during the isothermal expansion, and rejected at a uniformly low temperature, Tcold, during the isothermal compression. The maximum efficiency, ηmax, of these three cycles is: ηmax=1−(Tcold/Thot). This efficiency is attainable only if the engine is “reversible,” meaning that the engine is frictionless, and that there are no temperature or pressure gradients. In practice, real engines have “irreversibilities,” or losses, associated with friction and temperature/pressure gradients.
Reducing the size and complexity of any of these engines, while increasing efficiency, is important for a number of reasons. In most of the developing third world, adequate supplies of drinking water and water for irrigation are a scarce commodity. In many places in Africa, India and Central and South America, adequate supplies of water are found only at considerable depth below the surface. These locations generally do not have the infrastructure to provide an electrical grid to pump the water with electricity, nor do they have the infrastructure to provide roads to bring in electrical generators or even the fuel for those generators. Without an electrical grid, or without generators to generate electricity, maintaining a supply of potable water in isolated areas is a challenge, and refrigeration to keep medicine or foodstuffs from spoiling is unavailable. Even in the United States, there are communities such as the Amish communities where electricity is banned. Here the lack of cooling capabilities severely limits the production of various products. Because of the lack of cooling, milk production is limited to Grade B.
Consequently, there is a need for more efficient heat engine for converting the heat from the sun into mechanical energy, and ultimately for power generation, and a need for more portable systems.